A conventional object detection system radiates a laser light and receives a reflected laser light signal to detect objects such as preceding vehicles or to measure distances to objects as disclosed in JP 2002-22827A. This system uses a light receiver circuit shown in FIG. 3, for example.
The light receiver circuit includes a photo diode 72, a grounding resistor 74, an a.c. coupling capacitor 76 and an amplifier 78. The photo diode 72 is reverse-biased and allows a current to flow in the resistor 74. This current and the voltage across the resistor 74 changes with the amount of light incident from the surrounding environment. The changes in the light signal, that is, an a.c. signal corresponding to the reflected laser light is passed to the amplifier 78 through the capacitor 76 to be amplified and then signal-processed.
If the photo diode 72 directly receives sun light, which is a background light, a large current flows in the photo diode 72. In this situation, if the resistance of the resistor 74 is large, the voltage, that is, d.c. signal, produced by the resistor 74 undesirably becomes high. As a result, the diode 72 cannot be sufficiently reverse-biased and the receiver circuit cannot operate properly to detect a changing light such as the reflected laser light in the background light.
If the resistance of the resistor 74 is small, on the contrary, the voltage produced by the resistor 74 and indicative of the background light can be held low. Changes in the light received by the photo diode, that is, a.c. signal, passed to the amplifier 78 also become small. Therefore, it becomes difficult to detect changes (reflected laser light) in the received light. Further, if the resistance of the resistor 74 is small, thermal noise increases. As a result, signal-to-noise (S/N) ratio becomes unacceptable.